Descriptions

Three stem rust Puccinia graminis graminicola, resistant sources of perennial ryegrass, Lolium perenne, 4A, 48A, and 77A, were crossed in a partial diallel design with one susceptible source, MP-2, resulting in six crosses excluding reciprocals. The F1's and cloned parental populations were inoculated in the field and rated on a stem rust scale of 1-9 with 9 best (immune), once a week for five weeks. Maturity stage ratings of 1-6 with 1 during the vegetative stage and 6 at full maturity were also taken once a week. Maturity stages were not highly correlated with stern rust reactions except during the bootstage and early anthesis. Genetic differences were found between parental sources for stem rust reactions. Frequency distributions based on stem rust reactions among the F1 progeny populations suggested that resistance was quantitatively inherited with minor and possibly some major genes acting in an additive manner. Transgressive segregation toward susceptibility and slow rusting types were evident. Broad sense heritability estimates for stem rust resistance varied depending on the particular cross. The estimates suggested that there is considerable genetic variability for resistance between the parental lines. Selection would be more effective in the bootstage stage prior to anthesis when the heritability estimates were found to be higher. As the inoculum increased in the later stages of plant development, the resistance appeared to be overcome by the spore load. Seedling stem rust reactions were found to differ from adult plant reactions in the F1 generations of the six crosses in this study. In the F2 generation with open pollination of the F1 parents a gradient of percent susceptible seedlings occurred. Results of this study indicate a potential for developing stem rust resistant varieties with several minor and some major genes for increased longevity of resistance.